Intermetallic lithium-magnesium hexamethyldisilazide: Synthesis and structure, discovery of an oxygen-centered variant, and a reaction with benzonitrile that produces a novel amidinate cage compound with a trigonal bipyramidal Li4MgO core

Intermetallic lithium-magnesium hexamethyldisilazide [{(Me3Si)2N}3LiMg], 1, has been synthesized from a nonstoichiometric 2:1 ratio of LHMDS and Mg(HMDS)2 in hexane/toluene solution. Its crystal structure reveals a simple dinuclear arrangement based on a near-planar NMgNLi ring: the Mg center occupies a distorted trigonal planar environment of three N atoms, while the Li center occupies a distorted tetrahedral N2C2 environment due to two additional C-Li interactions with the -N(SiMe3)2 substituents. This structure appears to be retained in arene solution on the basis of the evidence of 1H NMR spectral data. In the presence of trace amounts of oxygen, the surprising peroxide-oxide variant [{(Me3Si)2N}4 Li2Mg2(O2)x(O)y] preferentially crystallizes from solution. X-ray crystallography has established that its structure contains peroxide predominantly [x = 0.715(7); y = 0.285(7)], the presence of which induces an aggregation state increase relative to 1: the four indistinguishable metal centers, held in a square-planar arrangement, side-on-coordinate to the peroxide molecule and bind to two N atoms. The oxygen-scavenging ability of this system has been exploited further in a reaction with benzonitrile. This has yielded a third intermetallic lithium-magnesium crystalline compound, having the composition [{PhC(NSiMe3)2}4Li4Mg(O)]. X-ray crystallography has revealed its remarkable cage structure built around an unprecedented distorted trigonal bipyramidal Li4MgO core, at the center of which lies a 5-O atom. Interestingly, all the metal centers lie out of the benzamidinate NCN ligand planes to indicate a significant degree of -bonding. One NCN ligand also adopts an unusual face-capping mode to a Li3 triangle, the like of which has previously only been observed in transition metal cluster compounds.